As a method of producing a hat-shaped steel, which is one type of shaped steel, press forming using a punch and die is widely known. In bending into a hat shape by press forming, the problem of springback, that is, the sheet material trying to return to its original state due to the reaction force when the press pressure is removed, easily arises, and therefore in the past, countermeasures for suppressing springback have been studied.
In this regard, in recent years, use of high tensile steel has been expanding. As one example, in the automobile industry, it is believed that reduction of the weight of the vehicle body will lead to reduction of the amount of emission of CO2 and therefore high tensile steel is being proactively used for the vehicle body material. For this reason, on the production floor of shaped steels, the problem of the springback due to the high strength characteristics of steel materials has been surfacing. Furthermore, in recent years, high tensile steel which has an over 980 MPa tensile strength has also been being produced. With general press forming, it is difficult to produce a hat-shaped steel as designed from such high tensile steel.
As another method of producing a shaped steel, the roll forming method is known. Roll forming is, for example, a continuous bending process which runs a strip, which is taken out from a coil, through roll units provided at a plurality of successively arranged stations. Roll forming is, in particular, suitable for forming H-beams, L-beams, and other steel products and pipes and other long products with constant cross-sectional shapes in the longitudinal direction. On the other hand, roll forming, unlike press forming (drawing), is not suited for forming a shaped steel which varies in cross-sectional shape in the longitudinal direction.
PLTs 1 to 3 disclose the art of roll forming to produce a shaped steel which varies in cross-sectional shape in the longitudinal direction by variable control of the roll widths of split rolls. However, the roll forming process and apparatus disclosed in PLTs 1 to 3 have the problem of a complicated structure and method of control of the apparatus. For this reason, it is difficult to convert existing facilities for use for working the inventions of PLTs 1 to 3. Introduction of new facilities is necessary, and therefore the cost becomes high.
Further, if, as in the inventions of PLTs 1 and 3, broadening the roll widths of the split rolls during roll forming, there are the problems that only the corner parts at the front sides of the rolls will linearly contact the steel sheet material and, in high tensile steel or other materials, stiffness of a mill is insufficient, and therefore it is not appropriate for mass production.